Currently, substantial resources are devoted to plans for cleaning river sediments of PCB's and other pollutants. The Hudson River, for example, has an EPA supervised PCB clean-up underway.
The following information is taken from public EPA materials (see www.epa.gov/hudson/pcbs101.htm). PCBs or polychlorinated biphenyls are a group of chemicals consisting of 209 individual compounds. PCBs were widely used as a fire preventive and insulator in the manufacture of transformers and capacitors because of their ability to withstand exceptionally high temperatures. PCBs were banned by the EPA in 1979, and are classified as a probable human carcinogen by numerous national and international health-protective organizations, such as the EPA, The Agency for Toxic Substances and Disease Registry (an arm of the U.S Public Health Service) and the World Health Organization. Research also links PCB exposure to developmental problems.
PCBs build up (bioaccumulate) in the environment, increasing in concentration as you move up the food chain. This is of special concern in areas where fish are exposed to PCB contamination and may be consumed by humans (as in the Hudson River).
There are 209 varieties of PCBs, known individually as congeners. A congener may have between 1 and 10 chlorine atoms, which may be located at various positions on the PCB molecule. For example:                monochlorobiphenyl=1 chlorine atom per molecule        dichlorobiphenyl=2 chlorine atoms per molecule        trichlorobiphenyl=3 chlorine atoms per molecule        tetrachlorobiphenyl=4 chlorine atoms per molecule        pentachlorobiphenyl=5 chlorine atoms per molecule        hexachlorobiphenyl=6 chlorine atoms per molecule        heptachlorobiphenyl=7 chlorine atoms per molecule        octachlorobiphenyl=8 chlorine atoms per molecule        nonachlorobiphenyl=9 chlorine atoms per molecule        decachlorobiphenyl=10 chlorine atoms per molecule        
A major problem is presented in that PCB's must be removed without causing further disbursement of the PCB's in the environment such as in the Hudson River. PCB's travel in river currents as they have an oil-like or sludge-like composition wherein they form a malleable and flowing sludge. However, PCB's are heavier than water and eventually settle in riverbeds as a sludge.
For example, one gallon of water weighs about 8.32 pounds whereas one gallon of PCB's weighs about 11.98 pounds. Riverbed silt is typically even heavier than the PCB's.
Traditional dredging techniques are not perfectly suited to handling and containing PCB's. Additionally, known methods of targeted pick-up of PCB's have deficiencies, for example “environmental dredging” wherein a grid with GPS coordinates is used is only effective for a short time period because other PCB sludges eventually migrate back to the site which was “cleaned.” Therefore, a suitable PCB capable sediment filter is needed, and preferably a filter which can remain in its location for long periods of time, but that is also removable if needed and that does not impede river traffic or activities.
The transportation of sediments by the current of a waterway is determined by the properties of the sediment particles and the velocity and force of the current. The most important thing influencing the distance sediments can travel is the fall velocity. Fall velocity is experienced when a particle or particles is suspended and transported by a force, (in this case the natural current of the waterway). When the particle reaches a point were the suspending force acting on it is less than the gravitational force, the particle falls due to gravity. Differences in inertia between materials, density and shape also influence the fall velocity or settling rate.
Non-cohesive materials such as sand and limestone are formed as a loose collection of small particles. Cohesive materials are clay and mud which build up sediment flakes or flocs. Sediment flakes or flocs settle fast because of their weight and inertial difference compared to water. PCB's tend to form cohesive oil-like or sludge-like flakes.
Suspended sediments occur in various forms. In low sediment concentrations, the water can be regarded as water containing individual settling particles. In higher sediment concentrations, thick mud develops with water and sediment to form a homogenous mixture. This mixture sinks to the bottom of the riverbed, from there it spreads out horizontally (density flow). The heavy mud consolidates slowly and is difficult to filter. PCB's are carried in the sediments.
Dredging Methods
The following is public information cited from IHC Holland company website (see IDS). For the last decade, engineers have introduced many different methods to dredge rivers and canals. During this period, the main focus for dredging was and still is to increase the depth and width of the rivers and canals to accommodate the passage of vessels. Dredging causes turbidity in surrounding waters and, depending on the local currents, a large part of the dredged material can spread over a larger area. In the case of dredging polluted sediments, the impact on the environment due to turbidity and its dispersal can be disastrous. Polluted material like silt, mud or soft clay can be removed using different types of dredging equipment. The contaminated silt that is found in the river results from factories or plants near the banks of these rivers. In estuaries, where the sea and fresh water meet, sedimentation by flocculation will occur, accumulating the polluted materials.
Dredging Equipment
As previously indicated, the suitability of a type of dredger depends on the minimizing of turbidity, high accuracy in positioning the digging element, or suction entrance and the ability to remove the soil at the highest possible density. Briefly, representative types of dredging will be discussed below with indications of their application.
Trailing Suction Hopper Dredger
Very well suited to dredge soil like silt, mud or soft clay, which is usually in layers on top of materials of higher density like stiff clay. Accurate positioning of the drag head in the horizontal plane is difficult, indeed almost impossible. Vertical positioning is also difficult unless low densities are accepted or special equipment is installed. The separate dredging of soil of different contamination classes in small dredging sites is very difficult and use of a trailing suction hopper dredger is therefore limited in these situations. Turbidity of surrounding water is very high, depending on the acceptance of overflowing and the type of overflow system used, on the use of a light mixture overboard system, the trailing speed and on the rate of maneuvering.
Bucket Dredger
This is suitable to dredge a wide range of materials. The positioning of the bucket in both the horizontal and vertical plane is very accurate. A major problem is encountered when thin layers of contaminated soils are dredged above a non-polluted bed. For instance, if a bucket dredger with bucket of 800 liters is used to remove a silt layer of only 40 cm, the density of the dredged material will be very low, due to the size of the buckets. Turbidity of the surrounding water, even at low bucket speed, is high but in general lower than for a trailer. For consolidated and also polluted sediments like clay, it is probably the best solution.
Cutter Suction Dredger
This is suitable to dredge more or less the same soil as indicated for the bucket dredger. The positioning of the suction mouth is rather accurate but in soft materials it is less effective in dredging soil outside of the cutter range. Turbidity is very high with a cutter at work and low when the cutter is stopped, but in this case the density of the soil dredged is also very low. If possible, reduction of the cutter speed to minimize turbidity, will result in lower mixture densities unless low mixture velocities are accepted and permitted. The use of cutter dredgers is only recommended when soil are very cohesive turbidity is less important and when density and/or mixture velocities of the pumped material are of minor importance in a given situation.
Stationary Dredger
Due to the basic principles of this type of dredger it is only suitable for dredging contaminated silt in very thick layers, for instance deposits in deep pits. Positioning of the suction mouth rather poor but is not important when the dredger is working in silt trap with silt layers of 2 to 5 meters. This is unable to handle cohesive soils unless a jet system is installed at the suction mouth, but in such a case then turbidity of surrounding water is very high.
Grab Dredger (with Grab on Wires)
The positioning of grab dredger in the horizontal plane is very accurate in shallow waters without currents. Positioning in the vertical plane is very poor in currents and flat bottoms can not be produced by dredging unless special precautions are taken to compensate the vertical movement of the cutting parts of the grab itself when it is closed. If a fully closed grab is used, turbidity of surrounding water is minimal. In combination with a so-called “Silt Curtain” a grab dredger can provide a practical solution from an environmental point of view. Grab dredger (with grab hanging on controllable arm).
However vertical and horizontal positioning in excellent and dredging a flat bottom with the aid of microprocessor-controlled positioning system is very good. The unit permits accurate dredging in tidal areas with low moderate currents. Environmentally a very good solution. Unless high investments are made however, production is limited.
Backhoe Dredger
If special arrangements are made to close the bucket when it is filled with soil and before the bucket is moved to the surface. The fitting of an effective and safe closing device, however, will be a weak point in this system if the dredging site is contaminated by stones, wires, anchors, etc.
Dustpan Dredger
Dustpan dredgers are only suitable for on flat bottoms with thin layers of silt. In order to minimize turbidity the use of jet water is not recommended, but jetting water is required to draw in the silt over the full breadth of the suction mouth. Positioning in the vertical plane is good, while positioning in the horizontal plane is moderate, depending on the anchoring system. This type of dredging is not suitable for cohesive materials.
Wheel Suction Dredger
From an environmental point of view this is similar to the cutter dredger. Due to the larger diameter of the dredging wheel, turbidity in surrounding water is somewhat worse than for cutter dredgers.
Mechanical Agitation Dredging
A very simple system (FIG. 7) with low investments used to transport limited amounts of silt from the sides of a channel or harbor basin into deeper water. Usually the system is used to supplement other dredging equipment. The positioning of the system in the horizontal plane is very limited. Separate wires provided running to a hoisting gantry on aft side of the tugboat to adjust the dept of the agitation device. A high rate turbidity occurs if it is used in shallow waters due to propeller wake. If the system is used in deeper water, the accuracy of vertical positioning is limited, but at lower tug speed the turbidity of surrounding waters is rather low.
Agitation Dredging by Means of Water Injection
Such system is based on the principle that fluid silt behaves like water and run down a slope when it is fluidizer. When water is injected into the silt, the silt will run from the sides of the channel or harbor basin to deeper water where it can be dredged. Positioning horizontal and vertical plane is rather good, but penetration depth silt depends on the type of waterjets used and density and nature of the silt, which makes this dredging system complicated and the positioning accuracy worse.
Agitation dredging must not to be confused with the normal way of agitation dredging where turbidity is essential to transport the sol by means of the tidal (or river) current. This system cannot be used to remove polluted soil due to the excessive turbidity of surrounding waters.
Other Systems Related to Dredging
Besides the mentioned equipment normally used in dredging silt and other low-cohesive soil, other types of system have to be mentioned to complete this list of dredging equipment. They are more or less additional tools and of a more stationary character.
Silt Trap
A silt trap is a deepened part of the harbor or estuary arranged to strategic place where silt is trapped due to excellent settling conditions. The silt will accumulate in a thick layer below the nautical depth and can than be dredged at high production rates at low cost by normal dredging equipment like trailing suction hopper dredgers. The main feature of this system is to catch, as far as possible, polluted silt at one place where it can be dredged under the best environmental conditions. This can be reached for instance by means of a silt trap with a sloping bottom to a deep part of the trap outside the traffic lane where it can be dredged with the best environmentally acceptable dredger available. Depending on the capacity of the silt trap, the gap between two successive maintenance periods can vary from almost continuous to once a year or less.
Stationary Silt Pumping Station
In addition to the silt trap, tests have been done in the past to place a fixed suction mouth (or numbers of suction mouths) in the lower part of a silt trap with a stationary pumping station. The aim is to pump the silt at a constant and low production rate to a disposal site via a pipeline. Due to the low pumping capacity, pumping 24 hours a day, is needed to remove the silt and the absence of moving parts like cutters, this system can be a good solution for dredging silt in rivers with constant delivery of polluted silt at the docks or the estuary.
Silt Curtain(s)
Silt curtains are an additional tool for stationary systems like grab dredgers, backhoe dredgers, and stationary dredgers, wherein the turbidity is strictly limited to the inside of the curtain, reaching from the water level to the bottom. This system is positioned beside or in front of the dredger, or floating separately held in position by anchors. Depending on the shape and construction of the unit, the workable current is limited to approximately 0.3 metros per second.
The above mentioned list equipment is not complete but it contains most of the equipment used in dredging today. During the last decade a large number of small, special dredgers have being designed for dredging (and often also for processing) polluted silt. Most of them are design for special conditions of very small capacities and special type of pollutants and some have not been built. Well-proven systems, of which more than one has built, are very exceptional and only built on a small scale. To date information from impartial experts about these systems is not available.
Transport Systems
A short note about transport of the dredged material is included below. Basically there are three possibilities: transport by means of dredger itself (for example, a trailing suction hopper dredger), by means of a discharge line (for instance with a cutter suction dredger) or by means of barges (for instance loaded by means of a bucket dredger or grab dredger). The barge can be emptied by means of a grab, a pump or by dumping the material through bottom doors or by splitting the hull. If barges, or trailing suction hopper dredgers transport contaminated silt, special care has to taken to avoid spillage of the contaminated silt due to swell and waves during transport. Discharging of contaminated silt by barges or trailing hopper dredgers is recommended for non-or acceptable low-polluted silt only, unless dikes enclose special arrangements like silt curtains are provided at the disposal site or the site. If the contaminated silt is pumped to a storage pond, special precautions have to be made to trap the contaminants from the water flowing over the deposit weir.